More particularly, the invention relates to a mounting flange provided at each end of a transmission shaft, each mounting flange being provided with at least one flexible coupling, each coupling comprising two flexible disks.
Consequently, the invention lies in the technical field of transmitting power from one mechanical member to another, and in particular between mechanical members of a rotorcraft.
Most presently-constructed rotorcraft are fitted with at least one free turbine turboshaft engine. Power is then taken from a low pressure stage of the free turbine, which stage is mechanically independent from the compressor assembly and from the high pressure stage of the turbine engine. The free turbine of a turboshaft engine generally rotates at a speed in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a rotary speed reducing gearbox is needed for the connection to the main rotor of the rotorcraft since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is the main gearbox (MGB).
Under such conditions, the turbine engine is connected to the main gearbox of the rotorcraft via at least one transmission shaft that rotates about its axis of rotation at a speed that is often greater than 5000 rpm.
Similarly, a rotorcraft generally has a tail rotor to compensate for the reaction moment due to mechanically driving the main rotor and serving to enable the rotorcraft to be steered.
Consequently, at least one power transmission shaft is generally arranged between the main gearbox and a tail speed-reducing gearbox for delivering power to the tail rotor. Such a transmission shaft can also be caused to rotate at a speed greater than 5000 rpm.
Given these speeds of rotation, the transmission shaft needs to be securely fastened by mounting flanges to the mechanical members it interconnects.
Furthermore, these mounting flanges must be capable of allowing the transmission shaft to transmit the power developed by one mechanical member to another under conditions that are extreme, specifically when the interconnected members are not in alignment with each other.
In order to combat the vibration generated by rotating assemblies, rotorcraft are provided with anti-vibration devices.
Consequently, the main gearbox, and possibly also the engine of the rotorcraft are fitted with anti-vibration devices. Thus, the gearbox and the engines are potentially free to move relative to the structure of the rotorcraft, in order to limit the vibration they generate.
The transmission shafts arranged between these mechanical members must thus be capable of operating in spite of these movements, in particular vertical movements that give rise to misalignments between the members concerned relative to one another.
Document FR 1 020 765 describes mounting flanges that accommodate that type of misalignment.
The mounting flange comprises a plurality of diaphragms of U-shaped axial section. Each diaphragm comprises two facing vertical elements that are provided with respective circular fasteners.
A first diaphragm, having a vertical element provided with a first circular fastener is then connected to a second diaphragm having a vertical element provided with a second circular fastener, by fastening the first circular fastener to the second circular fastener by means of nuts and bolts.
During movement of a mechanical member that is secured to the mounting flange, e.g. the engine of a rotorcraft, the mounting flange deforms so that the assembly comprising the mechanical member and the transmission shaft does not jam. The engine can then continue to drive the transmission shaft and thus the main gearbox, even in the event of the mechanical members being misaligned.
That mounting flange performs its function well. Nevertheless, it is found that devices making use of that principle sometimes present a lifetime that is limited. It is not unusual to find breaks in the diaphragm, particularly at the boundary between a vertical element and a circular fastener, e.g. after being used for about 8000 hours (h).
Given the cost of each mounting flange, such breaks are therefore expensive and harmful.
Since this drawback appears to be insoluble to the person skilled in the art, it is naturally necessary to make do with a lifetime of 8000 h.
The conventional knowledge of the person skilled in the art does not allow for using the relatively simple technique of increasing the thickness of each vertical element of each diaphragm of the mounting flange for the purpose of avoiding breakage. Increasing the thickness of the vertical element would make it stiffer.
Consequently, the movement of the vertical element would be modified and a fixed mounting flange therefore cannot take the place of the original mounting flange.